1. Field of the Invention
The present invention relates to the assessment of a signal as an input for an auditory device, and to the selection between possible inputs.
2. Related Art
Auditory devices include any acoustic or electrical auditory devices, such as hearing aids, middle ear implants, intra-cochlear implants, brain stem implants, implanted acoustic devices or any combination of these, for example devices providing combined electrical and acoustic stimulation. For those devices having an external device and an implanted device, the external device may be continuously, intermittently or occasionally in communication with the implanted device.
Auditory devices require, as an input, an electrical signal corresponding to an audio signal for processing in the device. This input is most commonly provided by a microphone. For example, a conventional cochlear implant consists of an external part containing a microphone, a sound processor and a transmitter, and an internal part which contains a receiver/stimulator device and an electrode array. Sound enters the microphone, which outputs a corresponding electrical signal to the sound processor, which in turn codes the sound using one of many possible processing strategies. The coded signal is sent to the transmitter, which sends it to the implanted receiver/stimulator unit. The receiver/stimulator sends the corresponding stimuli to the appropriate electrodes, so as to provide a percept of hearing for a user.
It will be apparent that the quality of outcomes for the user is dependant upon the quality of the detected audio signal. The quality of sound detected by a microphone can be unsatisfactory for the user particularly when in noisy environments, for example, when talking on the phone, or in movie theatres, sports stadiums, churches, and the like. This is generally due to a combination of factors including limitations in speech processing strategies, and the inability of such devices to overcome background noise, to detect signals at a distance from the source or to accurately convey the directionality of sound.
A telecoil acts as an alternate or supplemental input device to the auditory device, typically in noisy or difficult acoustic environments. A telecoil is a miniature receiver that picks up magnetic sound signals from telecoil-compatible phones and assistive listening systems (ALS). Telecoils are made up of a metal core around which ultra-fine wire is coiled. When the coil is placed in an electromagnetic field, a signal is induced in the wire. This signal can be used as an alternative input for the auditory devices.
Telecoils in many situations improve sound quality and allow different sound sources to be directly connected to the auditory aid. The telecoil is used to couple with the magnetic fields produced by telephones and hearing loops, assisting the user by tapping directly into a sound source and providing a clear signal free from audio background noise. In many situations, the telecoil signal may provide a clearer signal than the corresponding signal from the microphone on the user's auditory device. However, it will be appreciated that there may be noise or interference associated with the telecoil, so that in some situations the telecoil signal although present may not be the preferred input.
On most auditory devices, the input signal can be switched between the microphone and telecoil transducers. Such switching can be done via a user-operated manual switch, or automatically by the device itself. It is desirable to provide a feature in the device to decide when to select the telecoil input signal over one or more microphone input signals, that is, to choose the higher quality signal for the user.
One method for making such a decision employs a magnetically actuated switch to detect the presence of a static magnetic field such as that produced by the speaker in a fixed-line telephone. The primary limitation of this method is that it is incapable of detecting an induction loop signal such as that commonly found in cinemas, lecture theatres and hearing accessories, requiring manual switching in such cases. Such switches are also commonly not sensitive enough and require either careful placement very close to a phone handset or the addition of a supplementary magnet on the phone handset.
Another class of methods uses signal processing algorithms to distinguish whether an input telecoil signal contains a ‘useful’ signal such as speech or music and switches to it if such a ‘useful’ signal is detected. Measures of ‘usefulness’ have been made using standard methods for speech detection such as measures of signal amplitude, amplitude modulation depth and measures of amplitude variation in the spectral domain.
The main limitation of methods of this type is that they are not robust to changes in orientation or movement of the telecoil within a strong magnetic field. This is because the strength of the magnetic field detected by a telecoil is highly dependent on the orientation of the telecoil in that field. If the telecoil is oriented parallel to the magnetic field lines, it will pick up a high amplitude signal. If it is oriented perpendicular to the magnetic field lines, the signal amplitude will be relatively much lower. If the orientation of the telecoil is changed quickly with respect to a static magnetic noise signal, or if the direction of the magnetic field lines were to change, it is possible for the amplitude modulation of the detected signal to resemble that of a speech or music signal. If the wearer of a hearing device were to run, jump, dance or tumble, or if a mobile phone were brought near and moved around, it may cause such an amplitude modulation based signal analyzer to falsely trigger and erroneously select the telecoil signal.
Other input devices may be used for auditory devices. For example, two or more microphones may be provided for a device. These may be on the same side of the head, for example in different positions in the same behind the ear device, or disposed on different sides of the head or attached at various other positions on the user. Alternative inputs may also include radio or other wireless links. Similar issues arise for all such situations, in that a decision to use one or more possible inputs, or a combination of inputs, is required.